1. Field of the Invention
The present invention relates to a coil, in particular a high voltage coil, to be arranged in a slot in a stator or rotor of an electrical machine, and to a method for its manufacture.
2. The Prior Art
A coil which is arranged in slots in a stator or rotor in an electrical machine often comprises a bundle of a plurality of conductors located adjacent to each other, which are provided with a conductor insulation for insulation of the conductors from each other. The bundle may consist of several turns of one single insulated conductor. It may also consist of several separate insulated conductors, which are electrically connected to each other at their ends, usually parallel-connected.
The conductor bundle is surrounded by a main insulation for insulating the bundle against the machine slot. Because of the great demands which are placed on the main insulation as regards resistance to corona, the main insulation is to a considerable extent built up of mica. Normally, the main insulation consists of a wrapping of a sheet- or tape-formed insulating material consisting of a layer of small mica flakes overlapping each other and arranged on a carrier material, for example a woven glass fibre. An impregnating resin may be applied in and on the above-mentioned sheet- or tape-formed insulating material, for example by preimpregnating the insulating material with the resin which, when the insulating material is applied around the conductor bundle, binds the different layers in the main insulation to each other and to the conductor bundle. However, such a resin may instead be supplied after the insulating material has been applied around the conductor bundle and possibly also after several such insulated conductor bundles have been placed in the machine slots and been connected together to form a winding. The insulated conductor bundle as such, and when it is included in the winding, is then impregnated with an impregnating resin. Irrespective of in which way the impregnating resin is supplied, it is of the utmost importance that the impregnating resin to the greatest possible extent fills up voids in the wrapping of insulating material, so as to counteract the occurrence of corona discharges. The resin will then penetrate into the mica layers as well.
It is greatly desired that the main insulation has a good thermal conductivity because an increased thermal conductivity results in increased performance of the machine.
To improve the thermal conductivity of a mica insulation for electrical machines, Japanese patent application Laid-Open Print No. 53802/1980 proposes incorporating a powder of an inorganic material with a better thermal conductivity than the mica, such as boron nitride or aluminium oxide, into the mica layer together with synthetic fibres such as polyamide fibres, the task of the fibres being to serve as a reinforcing material for the mica insulation. The manufacture of the known mica insulation takes place by treating a suspension of small mica flakes, the powder of the inorganic material, and the synthetic fibres in accordance with a method similar to that used in paper manufacture while forming a sheetformed product. This causes the powder of the inorganic material, together with the synthetic fibres, to become embedded into the mica layer. When the mica insulation is later impregnated with an impregnating resin, the embedded powder particles facilitate the impregnation. In the known case, a size of the particles in the powder of the inorganic material of 30-100 .mu.m is used.
According to the present invention, it has proved to be possible, by incorporating inorganic fillers in a main insulation for a coil for an electrical machine, to increase the electric strength considerably while at the same time it has been possible to retain the other properties of the insulation, such as mechanical properties, thermal properties, internal corona level and dissipation factor as a function of temperature and voltage. According to the invention, this is achieved by incorporation of a powder of inorganic material having good thermal conductivity and with a size of the powder particles of 0.1-15 .mu.m.
More particularly, the present invention relates to a coil for arrangement in a slot in a stator or rotor of an electrical machine, comprising a bundle of a plurality of conductors arranged adjacent to each other and provided with a conductor insulation for insulation of the conductors from each other as well as a main insulation surrounding the bundle for insulating the bundle against the machine slot, the main insulation comprising a wrapping of a tape- or sheet-formed insulating material wrapped in several layers around the bundle and comprising a layer with small mica flakes which, as well as the spaces between layers of mica flakes in the wrapping, contains a cured impregnating resin, characterized in that at least that part of the impregnating resin which is located in the spaces between mica layers contains particles of a filler having an intrinsic thermal conductivity of at least 5 W/mK (Watts/meter.degree Kelvin) and with a size of the particles of 0.1-15 .mu.m in at least 90 percent by weight of the filler. Preferably, the intrinsic thermal conductivity of the filler amounts to at least 10 W/mK and the size of the particles amounts to 0.7-5 .mu.m in at least 90 percent by weight of the filler.